Respiratory oxygen is delivered to patients in both sub-critical care situations in which oxygen is provided as a supplement to room air that may be inhaled by the patients (often referred to as "supplemental" oxygen delivery) and in critical care situations in which the gases (particularly oxygen) inspired by patients are closely controlled, possibly in connection with blood gas measurements.
Continuous oxygen delivery in sub-critical and critical care is, however, wasteful because oxygen is only needed by patients when they are inhaling and the oxygen delivered at other times is wasted. In supplemental oxygen delivery, the most significant financial cost associated with this waste is found in the increased service visits required by the oxygen provider to replenish the patient's oxygen supply, because the actual cost of the oxygen is only a small fraction of the total cost of the therapy. In critical care environments, the capacity of the oxygen delivery system must be increased to account for the oxygen delivered when the patient is exhaling.
One approach to conserving the oxygen delivered to patients is typically referred to as "demand delivery." The demand delivery systems respond to a patient's inspiratory effort by attempting to deliver oxygen during the period of inhalation, rather than allow the oxygen to flow to the patient continuously. There are many ways in which this basic concept has been implemented.
A variety of respiration detection systems have been developed in connection with demand delivery systems. Examples of some respiration detectors include a chest belt worn by the patient that generates an electrical signal to trigger the opening of the oxygen supply valve; a hand-activated breathing device attached to a portable gas bottle via a supply hose in which users dispense the oxygen by pushing a button while holding the device next to their mouth; a mechanical chest strap/valve that functions as both an inhalation sensor and delivery device in an oxygen conserving system; and an all-pneumatic, fluidically-controlled device.
Other demand delivery systems use pressure sensors in the oxygen line to monitor line pressure at the nostrils. A small negative pressure, indicative of the onset of inhalation, is often relied on to trigger the release of oxygen. This type of detection scheme has become a standard method of demand delivery and is employed by most systems currently in use.
Yet another approach employs flow sensors to monitor patient respiration. In many of these demand delivery systems, the flow sensors are located in-line with the source of oxygen such that, in addition to monitoring patient respiration, the flow sensors also monitor the amount of oxygen passing through them during the oxygen delivery portion of the cycle. It is, however, difficult (if not impossible) to accomplish both of those functions with a single flow sensor because the flow rates that need to be sensed differ by orders of magnitude. For example, when measured using a nasal cannula, patient respiration may result in flow rates at the flow sensor of about 1 cubic centimeters per minute (cc/min) or less, while the flow rate of oxygen delivered to the patient can be up to about 15 liters per minute (l/min) or more. One significant problem is in sensing inhalation or exhalation simultaneously with the delivery of oxygen in these situations. Examples of this approach are found in U.S. Pat. No. 4,823,788 (Smith et al.) and U.S. Pat. No. 5,558,086 (Smith et al.).
In addition to the above-listed problems, even if the flow sensor has a dynamic range capable of sensing respiration and delivery oxygen, the systems do not allow for correction in the "drift" often associated with such sensors. In other words, over time the accuracy of the sensor may be impaired because of dynamic changes in the flow sensor during use. Because the flow sensors are always in use, monitoring either oxygen flow or respiration, adjustments are difficult if not impossible to make to correct for voltage drift.
Another flow sensor-based demand delivery system is described in U.S. Pat. No. 5,074,299 (Dietz) in which a flow sensor is connected to one port of a nasal cannula while oxygen is delivered through the other port of the cannula. One disadvantage to this approach is that if one side of the patient's nasal cavity is blocked due to an upper respiratory infection or cold, the system will either be unable to effectively deliver oxygen or sense respiration (depending on which port of the cannula is located on the blocked side).
Even if the patient is not experiencing blockage, it may be difficult for the sensor to detect exhalation if the patient is breathing through his or her mouth. The nasal respiratory flow rate is significantly reduced in such patients, and if flow sensing is occurring only through one port in the cannula, it may be too low to be accurately detected. Another disadvantage is that the system requires a more expensive and obscure dual-line cannula.
Regardless of whether pressure or flow sensors are used to detect respiration, shallow respiration can make respiration sensing in systems relying on pressure/flow sensors difficult or impossible because the flow volume and/or flow rate of gases associated with respiration deteriorate as the gases from inhalation or exhalation travel through the lines connecting the patient's respiratory system to the sensor. The flow volume/rate deterioration caused by the lines further reduces the flow volume/rate produced by a patient engaging in shallow respiration. As a result, the respiration sensor may be unable to detect respiration or may detect only a portion of the actual inhalation/exhalation events.
Another problem associated with patients experiencing rapid respiration rates is that some demand delivery systems include a time delay period to prevent sensing of the same inhalation period more than once. These systems operate on the assumption that each inhalation lasts for a predetermined minimum amount of time and the system is prevented from sensing inhalation within the delay period. Because the patient is, however, experiencing rapid respiration, the patient may actually inhale two or more times during the delay period. The effect of such a "lockout" feature is that the patient will receive oxygen during only a fraction of their inhalation events. In addition, if the respirations are rapid and the system delivers oxygen too slowly, the patient may already be exhaling by the time the system begins to deliver oxygen.